1. Field of the Invention
This invention relates to a process for preparing maleic anhydride from a hydrocarbon mixture containing C.sup.5 monoolefinic hydrocarbons, C.sub.5 diolefinic hydrocarbons (cyclic and acyclic), C.sub.5 paraffins, C.sub.6 paraffins and benzene wherein the C.sub.5 cyclic diolefinic hydrocarbons have been removed and the resulting mixture in vapor phase is reacted with molecular oxygen in the presence of an oxidation catalyst at elevated temperatures.
2. Description of the Prior Art
The catalytic oxidation of hydrocarbons. alone or mixtures thereof, to produce a product containing maleic anhydride is known. Included among the hydrocarbon charge stocks are aromatic hydrocarbons, such as benzene, toluene, ortho-xylene etc.; monoolefinic hydrocarbons such as butene-1, butene-2, etc.; diolefinic hydrocarbons, such as 2,3-dimethyl butadiene, 1,3-butadiene, isoprene, 1,3-pentadiene, etc. This is shown, for example, in U.S. Pat. Nos. 3,156,705 to Kerr 3,288,721 to Kerr and 3,538,122 to Friedrichsen et al. In some cases these charges have been obtained from cracked naphthas, as, for example, in U.S. Pat. Nos. 2,719,853 and 2,773,838 to Reid et al.
Naphtha is a volatile liquid hydrocarbon mixture produced by the fractional distillation of petroleum with a boiling range at from about 27.degree. to about 260.degree. C. comprising acylic and cyclic paraffins and olefins and aromatic hydrocarbons. A significant amount of light olefins, for example, ethylene and propylene, are obtained by way of steam cracking of naphtha employing fired tubular furnaces operating at short residence time, high temperature and low hydrocarbon partial pressure. In addition to the light olefins, hydrogen, methane, ethane, propane, C.sub.4 paraffins, olefins and diolefins and a host of higher (normally liquid) hydrocarbons are also produced. In processing the product, water (from steam) is separated from the hydrocarbon mixture, then light (normally gaseous C.sub.1 -C.sub.4) hydrocarbons are separated from heavier (normally liquid C.sub.5 +) hydrocarbons and finally the mixtures are processed to separate and purify desired product (s).
Of the light hydrocarbons, ethylene and propylene are most desirable and are therefore purified for sale as chemical feedstocks. Hydrogen and methane are used for fuel, while ethane and propane are recycled to the naphtha cracker. The butadiene and butylene in the C.sub.4 fraction are separated and sold as chemicals or as synthetic rubber feedstocks. The remaining C.sub.4 paraffins are used as fuel or recycled to the naphtha cracker.
The liquid hydrocarbon product consists essentially of a mixture of C.sub.5 through C.sub.9 hydrocarbons having a boiling point at atmospheric pressure of about 24.degree. to about 204.degree. C. Normally, the liquid hydrocarbon product is separated into C.sub.5 hydrocarbons, C.sub.6 -C.sub.8 hydrocarbons C.sub.9 + hydrocarbons. The C.sub.6 -C.sub.8 hydrocarbons therein are mainly aromatics, for example, benzene, toluene, xylenes, etc., commonly referred to as "BTX components".
The mixture of C.sub.5 hydrocarbons contains a significant amount of BTX components and other C.sub.6 hydrocarbons, for example, n-hexane. Their presence in the C.sub.5 hydrocarbon mixture is due to the fact that in order to minimize the presence of C.sub.5 's in the BTX component mixture when the liquid hydrocarbon product is separated some C.sub.6 's are taken overhead with the C.sub.5 's.
The mixture of C.sub.5 hydrocarbons also contains a significant amount, for example, from about 20 to 25 weight per cent of isoprene. Since isoprene is valuable for the synthesis of polyisoprene, isoprene is removed from the C.sub.5 mixture. One separation technique involves concentration of the isoprene and then extraction of the same from the concentrate.
The C.sub.5 mixture remaining contains primarily C.sub.5 monolefinic hydrocarbons, for example, pentene-1, pentene-2, 2-methylbutene-1, 2-methylbutene-2, 3-methylbutene-1, cyclopentene, etc; C.sub.5 diolefinic hydrocarbons (cyclic and acyclic), for example, 1,3-pentadiene, 1,4-pentadiene, isoprene, cyclopentadiene, etc.; C.sub.5 paraffins, for example, 2-methylbutane, n-pentane, cyclopentane, etc., C.sub.6 paraffins, for example, n-hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, etc., and benzene.
Such a commercial mixture is defined below in Table I.
TABLE I ______________________________________ Compound Weight Per Cent ______________________________________ Pentene-1 &lt;0.1 2-Methylbutene-1 &lt;0.1 Isoprene 0.7 Isopentyne &lt;0.1 n-Pentane 0.4 -Pentyne-1 &lt;0.1 Pentyne-2 0.5 2-Methylbutene-2 0.7 Cyclopentadiene 39.7 1,3-Pentadiene 7.5 Cyclopentene 8.2 Cyclopentane 2.4 n-Hexane 28.4 Benzene 10.6 ______________________________________
Reference is made in the above table to cyclopentadiene, but such terminology herein is also intended to include its dimer dicyclopentadiene, since the two can be in equilibrium with each other, depending on the conditions in which each finds itself.